IEEE 802.11 Enhanced Distributed Channel Access (EDCA) is a method of medium access defined in IEEE 802.11e and is an enhancement to the original IEEE 802.11 Media Access Control (MAC) sublayer. EDCA provides four prioritized queues for data transmission. The queues use a contention-based mechanism to determine the next packet for transmission. The queue parameters are set such that the high priority queues have a preference for access to the wireless medium.
When the size of the MAC protocol data units (MPDUs) being transmitted for each class of traffic are approximately the same, the contention mechanism works well. However, when the MPDUs are of different sizes, the larger MPDU transmissions end up taking more medium time. For a device that is transmitting small voice MPDUs and large best effort MPDUs, the best effort traffic will introduce delays in the transmission of the voice traffic.
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